print repr(sys_version)
returns
'2.6.1 (IronPython 2.6.1 (2.6.10920.0) on .NET 2.0.50727.3603)'
The format of sys.version now start with a version number
2.6.1 (IronPythons 2.6.1 (2.6.10920.0) on .NET 4.0.30319.1)
My guess is that with previous version of IronPython this was not the case